RF filters are commonly used in RF communications circuits to remove signals at unwanted RF frequencies. An RF receiver may use RF filters to remove unwanted signals that have been received through an antenna, or signals from other circuitry, such as an RF transmitter. Some communications systems, known as full duplex systems, may transmit and receive signals simultaneously. Other communications systems, known as half duplex systems, may transmit and receive signals, but not simultaneously. Multimode systems may combine full duplex and half duplex modes of operation from different frequency bands. In a full duplex mode of operation, the transmit frequency is separated from the receive frequency to prevent interference between transmitted signals and received signals. The difference between the transmit frequency and the receive frequency is known as the duplex frequency.
RF receivers typically use RF bandpass filters to remove unwanted signals; however, in a system with a low duplex frequency, RF bandpass filters may be inadequate. Complex filtering may be required to remove transmit signals from the receive path. A common type of bandpass filter circuit is a surface acoustic wave (SAW) filter; however, in a multimode system, several SAW filters may be required to provide adequate filtering for multiple frequency bands. Some communications systems use a feature called receiver diversity, which provides multiple receive paths. In systems with receiver diversity, additional SAW filters may be required. An alternate type of filter is called a notch filter, which blocks signals at a notch frequency and passes signals at all other frequencies. A notch filter with a notch frequency at the transmit frequency may be placed in the receive path. Such a filter may be very effective at blocking unwanted signals at the transmit frequency. Filters that block signals at transmit frequencies and pass signals at receive frequencies are known as duplex filters, which may allow elimination of some SAW filters in receive paths; however, errors in the notch frequencies may reduce the effectiveness of such filters. In systems with a low duplex frequency, blocking signals at the transmit frequency without attenuating signals at the receive frequency may be difficult. Thus, there is a need for an RF duplex filter that blocks signals at the transmit frequency and passes signals at the receive frequency in systems with low duplex frequencies.